1. Field of the Invention
The present invention relates to the circuit configuration and layout of a pixel area of an active matrix display device in which thin-film transistors are used and source lines formed above gate lines. In particular, the invention relates to the structure of an auxiliary capacitor.
2. Description of the Related Art
In recent years, techniques of forming thin-film transistors (TFTs) on an inexpensive glass substrate have been made rapid progress. This is because of increased demand for the active matrix liquid crystal display device.
In the active matrix liquid crystal display device, thin-film transistors are provided for respective ones of hundreds of thousands to millions of pixels that are arranged in matrix form and the charge entrance and exit to each pixel is controlled by the switching function of the thin-film transistor.
A liquid crystal is interposed between each pixel electrode and an opposed electrode, to form a kind of capacitor. Therefore, image display is realized by controlling the quantity of light passing through the liquid crystal panel by varying the electro-optical characteristic of the liquid crystal by controlling the entrance and exit of change to and from this capacitor with the thin-film transistor.
The capacitor having the above structure has a problem that since the voltage held by the capacitor gradually decreases due to current leakage, it changes the electro-optical characteristic of the liquid crystal and deteriorates the contrast of image display.
A common measure to solve the above problem is a configuration in which an additional capacitor called an auxiliary capacitor is provided in parallel with the capacitor including the liquid crystal and charge equivalent to charge that is lost due leakage etc. is supplied to the capacitor including the liquid crystal.
FIG. 1 is a circuit diagram of a conventional active matrix liquid crystal display device. The active matrix display circuit is generally divided into three parts: a gate driver circuit 2 for driving gate lines (i.e., gate lines, scanning lines) 4, a data driver circuit 1 for driving source lines (i.e., data lines, source lines or signal lines) 5, and an active matrix circuit 3 that is provided with pixels. The data driver circuit 1 and the gate driver circuit 2 are generically called a peripheral circuit.
In the active matrix circuit 3, a number of gate lines 4 and source lines 5 are provided so as to cross each other and pixel electrodes 7 are provided at the respective intersecting points. A switching element (thin-film transistor) 6 is provided to control charge that enters or exits from each pixel electrode 7. Selection is made between the top-gate thin-film transistor (the gate electrode is formed above the active layer) and the bottom-gate thin-film transistor (the active layer is formed above the gate electrode) in accordance with the necessary circuit structure, the manufacturing process, the required characteristics, and other factors. Further, as described above, to prevent a variation in pixel voltage due to leak current, an auxiliary capacitor 8 is provided in parallel with each pixel capacitor.
On the other hand, the conductivity of the thin-film transistor is varied by illumination with light. To prevent this phenomenon, it is necessary to cover each thin-film transistor with a light-interruptive coating (black matrix). The light-interruptive coating is formed so as to also cover the portions between the pixels to prevent color or brightness contamination between the pixels and a display failure due to a disordered electric field at pixel boundaries.
So, the light-interruptive coating assumes a matrix shape and hence is called a black matrix (BM). At first, in favor of advantages in a manufacturing process, the black matrix was provided over the substrate (opposed substrate) that opposes the substrate on which the active matrix circuit is formed. However, recently, because of the need for increasing the area of each pixel (aperture ratio), it is proposed to provide the black matrix over the substrate on which the active matrix circuit is formed.